The new complaint is the latest salvo in an ongoing legal battle between Tesla and the UAW, which has accused the company of utilizing a series of unlawful tactics to thwart unionizing.

In August, based on a separate UAW complaint, the NLRB found that Tesla had violated workers' rights by requiring them to sign a confidentiality agreement that could bar them from talking about their working conditions and safety issues at the California plant.

In October, the UAW filed yet another complaint with the NLRB accusing Tesla of firing 400 workers who supported the union's campaign. The electric carmaker is also facing lawsuits accusing it of discriminating against black, gay, and older workers. Tesla has denied those claims.

Williams told reporters that a decision on when to hold a unionization vote at the Tesla plant will not come until a new UAW president is seated later this year.

A challenge for the union and its organizers has been the high rate of worker turnover at Tesla.

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(Reporting by Daniel Wiessner in Albany, New York and Joe White in Detroit, Michigan; editing by Tom Brown and Alexia Garamfalvi)